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Theriogenology. 2000 Dec 1;54(9):1395-408.

Sperm cryopreservation of African catfish, Clarias gariepinus: cryoprotectants, freezing rates and sperm:egg dilution ratio.

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Fish Culture and Fisheries Group, Wageningen University, The Netherlands.


Methods for cryopreserving spermatozoa and optimizing sperm:egg dilution ratio in African catfish Clarias gariepinus were developed. Five percent to 25% DMSO and methanol were tested as cryoprotectants, by diluting semen in Ginzburg fish ringer and freezing in 1-milliliter cryovials in a programmable freezer. To avoid an excess of spermatozoa per egg, post-thaw semen was diluted 1:20, 1:200 or 1:2,000 before fertilization. Highest hatching rates were obtained by spermatozoa frozen in 10% methanol and post-thaw diluted to 1:200. Then, slow freezing rates (-2, -5 or -10 degrees C/min) to various endpoint temperatures (range -25 to -70 degrees C) before fast freezing in liquid nitrogen (LN2) were evaluated. Hatching rates equal to control (P > 0.05) were obtained by spermatozoa frozen at -5 degrees C/min to -45 to -50 degrees C and at -10 degrees C/min to -55 degrees C. In 3-step freezing programs, at -5 degrees C/min, the effect of holding spermatozoa for 0, 2 or 5 min at -30, -35 or -40 degrees C before fast freezing in LN2 was analyzed. Hatching rates equal to control (P > 0.05) were produced by spermatozoa frozen to, and held at, -35 degrees C for 5 min and at -40 degrees C for 2 or 5 min. Finally, frozen spermatozoa (10% methanol, -5 degrees C/min, 5-min hold at -40 degrees C, LN2, post-thaw diluted to 1:200) were tested in on-farm fertilization conditions. Again, no difference (P > 0.05) in hatching rate was observed between frozen and fresh spermatozoa. Cryopreservation offers utility as a routine method of sperm storage and management for catfish.

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